1. Field of the Invention
The present invention relates to a state determining apparatus for determining the state of a humidity sensor arranged in an exhaust system of an internal combustion engine for detecting the humidity within an exhaust pipe.
2. Description of the Prior Art
The applicant has already proposed a state determining apparatus for a humidity sensor of the type mentioned above, for example, in Laid-open Japanese Patent Application No. 2001-323811. The humidity sensor is arranged at a location downstream of an adsorbent for adsorbing hydrocarbons contained in exhaust gases in order to determine a deterioration of the adsorbent. The determination on the deterioration of the adsorbent relies on the characteristic of the adsorbent made of zeolite or the like that adsorbs moisture simultaneously with hydrocarbons, and on a proportional relationship of the ability to adsorb hydrocarbons to the ability to adsorb moisture. Thus, the humidity of exhaust gases after passing through the adsorbent is detected by the humidity sensor to determine a degradation in the ability of the adsorbent to adsorb moisture, i.e., a deterioration of the adsorbent. Specifically, the state determining apparatus sets a predetermined time required for the humidity detected by the humidity sensor to rise by a predetermined value, associated with a gradual rise in the humidity of exhaust gases passing through the adsorbent which adsorbs moisture in the meantime after the start of an engine in accordance with the humidity at the time of the start, and the like, and measures an actual time taken for the detected humidity to rise by the predetermined value. Then, the state determining apparatus determines that the adsorbent is deteriorated when the measured time is shorter than the predetermined time, from the fact that the detected humidity rises at a high rate.
While a deterioration of the adsorbent is determined based on the result detected by the humidity sensor as described above, a sensing element, which forms part of the humidity sensor, is exposed at all times to exhaust gases at high temperatures and therefore highly susceptible to deterioration. Since the deteriorated humidity sensor could adversely affect the accuracy of the deterioration determination on the adsorbent, an additional failure determination (state determination) is made for the humidity sensor together with the deterioration determination for the adsorbent. The failure determination for the humidity sensor relies on the fact that a value detected by the humidity sensor stably remains substantially at a constant value in a high humidity region when the adsorbent has completely adsorbed hydrocarbons and moisture in exhaust gases and is saturated. The humidity sensor is determined to be at failure when the humidity detected by the humidity sensor at a timing at which the adsorbent is estimated to be saturated is lower than a predetermined value.
However, the foregoing technique for determining a failure in the humidity sensor merely compares the humidity detected by the humidity sensor, which remains substantially at a constant value due to the saturated adsorbent, with the predetermined value. For this reason, for example, even when the humidity sensor is deteriorated with a lower responsibility which causes the detected humidity during adsorption of the adsorbent to actually change at a very low rate, the humidity sensor is determined as normal when the detected humidity eventually reaches the predetermined value. On the other hand, in the deterioration determination for the adsorbent, the adsorbent is determined as deteriorated when the humidity detected by the humidity sensor during adsorption rises at a high rate, as described above. Thus, when a deteriorated humidity sensor with a low responsibility is used for determining a deterioration of an adsorbent, the adsorbent, which is actually deteriorated, could be erroneously determined as normal because the detected humidity rises at a lower rate during adsorption, giving rise to a reduction in the accuracy of the deterioration determination for the adsorbent.